Hypoglycemia unawareness is a complication of intensive insulin therapy often encountered after episodes of iatrogenic hypoglycemia. The mechanisms by which the brain detects low blood sugar concentrations are uncertain. The brain contains 3-10 muol/g glycogen that may serve as a fuel during moderate hypoglycemia. Glycogen metabolism is insulin-and glucose-sensitive. Brain glycogen thus provides an aspect of cerebral carbohydrate metabolism that is sensitive to alterations in glucose homeostasis such as those seen in diabetic patients. The purpose of this project is to explore the potential role brain glycogen may have in modulating the symptoms of hypoglycemia. Upon completion of this project, we expect to have a better understanding of the role of brain glycogen during and following hypoglycemic episodes. The hypotheses of this project are a) That brain glycogen concentration and metabolism are modulated by plasma glucose and/or insulin concentrations in vivo. b) That the brain glycogen concentration serves as a reservoir of glucose equivalents that are used during hypoglycemia in vivo. c) That following a hypoglycemic episode, the brain stores more brain glycogen such that longer and deeper subsequent hypoglycemia is necessary to deplete brain glycogen, which may provide a mechanism leading to hypoglycemia unawareness, with the following specific aims: 1. To explore in vivo brain glycogen metabolism non-invasively using 13C NMR localization methods during hyperglycemia and at different plasma insulin levels. 2. To establish that brain glycogen serves as a reservoir of glucosyl units during hypoglycemia and to determine the control of brain glycogen following hypoglycemia and to correlate these changes with perfusion-based functional MRI (fMRI) measurements of cerebral blood flow changes. These aims will be achieved in rat brain using localized 1H and 13C NMR spectroscopy and perfusion-based fMRI.